The Research Institute of the Cleveland Clinic Foundation is a rapidly expanding research center with a growing core of NIH-funded investigators. In order to fulfill the urgent instrumentation needs of the Institute, we are requesting an analytical scanning transmission electron microscope, the Philips CM12/STEM, interfaced with an X-ray microanalysis system, the LINK AN10000. The Philips microscope was chosen based on its excellent vacuum system, ability to interface with a horizontal X-ray detector with a 20o take-off angle, ease of use in both TEM and STEM modes and simplified operating consoles. The principle reason that the LINK system was chosen was the exceptionally high resolution of the X-ray detector as well as the suitability of the analytical programs for on-line elemental quantitation of biological samples. Three out of eight of the primary users are investigating subcellular distribution of Ca2+ and other elements: during the cardiac cycle in hamster heart in vivo (Dr Bond), in axonal preparations from sciatic nerve in response to parathyroid hormone stimulation (Dr Breuer) and following exposure to oxidized low density lipoprotein during the S phase of the cell cycle in cultured human fibroblasts (Dr Chisolm). Each of these investigators requires the use of the X-ray microanalysis system, as well as the extended capabilities of the STEM, which will provide the opportunity to carry out X-ray mapping, in addition to spectral collection with a stationary probe in TEM. Dr Harasaki is a fourth user of X-ray microanalysis who will investigate elemental localization on the surfaces of bioprosthetic heart valves. Drs Hoff and Cole are investigating receptor-mediated endocytosis of low density lipoproteins, using colloidal gold labelled specimens, in some instances on specimens of low contrast. These investigators, together with Dr Shainoff, who is studying the role of fibrinogen in platelet aggregation, will greatly benefit from the enhanced image contrast available in STEM. Dr Block uses colloidal-gold labelled markers to examine the subcellular localization of the hypothalamic Angiotensin II system and Dr Jacobsen investigates the role of cobalamin-binding proteins in the enterohepatic circulation, using colloidal gold tagged antibodies to cabalamin-binding proteins. These two investigators will primarily operate the microscope in conventional transmission mode, but may also benefit from the STEM and from the particle counting software provided by LINK. The projects of Drs Hoff and cole, as well as Dr Harasaki, will also require the increased resolution provided in secondary and backscattered images that can be obtained in STEM (compared with SEM). In this mode of operation, they will utilize the image mixing capabilities of the microscope. In summary, a versatile, state-of-the-art analytical scanning transmission electron microscope will greatly benefit a large number of users in the Research Institute and will fulfill a much needed role as an Institute-wide facility.